1. Field of the Invention
The present invention relates to a Code Division Multiple Access (CDMA) system. More particularly, the present invention relates to a method and an apparatus for acquiring frame synchronization in a CDMA system.
2. Description of the Related Art
A Time Division Duplex (TDD) scheme is applied to a CDMA system, and the CDMA system based on the TDD scheme can use the same frequency band shared by both a downlink and an uplink and freely control and operate data capacities thereof by changing allocation of Time Slots (TSs) depending on the situation. Among such systems, Time Division Synchronous Code Division Multiple Access (TD-SCDMA) has been commercialized and is currently used in China.
FIG. 1 illustrates a structure of a sub-frame used in TD-SCDMA according to the related art.
Referring to FIG. 1, one sub-frame includes seven TSs having the same length for data transmission and a Special Time Slot (STS). The STS is used to distinguish between the downlink and the uplink, and includes a Downlink Pilot Time Slot (DwPTS), a Guard Period (GP), and an Uplink Pilot Time Slot (UpPTS). The DwPTS and the UpPTS include information for physical transmission synchronization. For example, a sequence for synchronization of forward link transmission is transmitted in a DwPTS field and a sequence for synchronization of reverse link transmission is transmitted in an UpPTS field. The TS in which data transmission is performed includes a data symbol interval and a mid-amble, wherein user data is included in the data symbol interval and pilot signal information for a channel estimation is included in the mid-amble. In general, TS0 is mainly used for data transmission for a broadcasting channel within a cell, and TS1 to TS6 are used for forward or reverse data transmission. D and U within each TS block refer to the downlink and the uplink, respectively.
FIG. 2 illustrates a structure of a DwPTS used for initial synchronization acquisition in a downlink according to the related art.
Referring to FIG. 2, the DwPTS includes GP and SYNC-DL codes. The SYNC-DL code is transmitted with constant power and is not spread. Power of a SYNC-DL sequence is determined by higher layer signaling.
When the CDMA system based on the TDD scheme is initially driven, initial synchronization acquisition is necessary. The initial synchronization acquisition generally includes three operations described below and may be embodied in various types according to the implementation.
Operation 1: Search of the DwPTS and Identification of the SYND-DL Sequence.
A SYNC-DL sequence index is determined by performing slot synchronization by using the SYNC-DL sequence in the DwPTS in a Second TS (STS) of one sub-frame. This is determined through a comparison between a threshold and a maximum value among 32×6400 hypotheses acquired by performing a correlation through a matched filter. At this time, it may be required to acquire a plurality of hypotheses in consideration of several cells and the number of effective multi-path for each cell.
Operation 2: Identification of Scrambling and Basic Mid-Amble Codes.
A corresponding mid-amble code group can be known from the SYNC-DL sequence acquired in the first operation, and a corresponding mid-amble code is identified among four basic mid-amble codes included in the group. The identified mid-amble code is equally used during one sub-frame. Further, since a relation between the mid-amble code and the scrambling code corresponds to a one-to-one correspondence mapping relation, when the mid-amble code is determined, the scrambling code is automatically determined. A correlation between the mid-amble code and the scrambling code is shown in Table 1 below.
TABLE 1Associated CodesCodeSYNC-DLSYNC-ULScramblingBasic Mid-ambleGroupIDIDCode IDCode IDGroup00 . . . 7001112233Group18 . . . 15442556677...Group31248 . . . 25512412432125125126126127127
Operation 3: Control Multi-Frame Synchronization.
A phase of a channel value through a mid-amble of a Primary-Common Control Physical CHannel (P-CCPCH) is reflected to the SYNC-DL, and a position of a control multi-frame is determined by using information on a phase of the DwPTS.
Operation 4: Broadcasting Channel (BCH) Decoding.
When a CRC becomes “Good” through demodulation of the BCH, all the operations are successfully completed and general communication starts. Such an operation may not be included in an initial synchronization process.
Meanwhile, in the CDMA system based on the TDD scheme, channels related to initial synchronization having the structure shown in FIG. 1 are periodically received. More particularly, since the SYNC-DL including 64 chips is received every 5 ms interval in TD-SCDMA, the TD-SCDMA system has relatively low detection capabilities in comparison with Wide-band Code Division Multiple Access (W-CDMA)/Frequency Division Duplex (FDD) systems in which initial synchronization is acquired using successively received pilot channels.
Meanwhile, since the BCH has a Transmission Time Interval (TTI) of 20 ms (one TTI=four sub-frames) in the TD-SCDMA system, a terminal should identify whether there is the BCH in a next frame and find a start position of the TTI. In order to inform of the start position of the BCH, a base station performs phase modulation based on the phase of the mid-amble of the first time slot (i.e., a time-slot 0) when modulating the DwPCH and a phase modulation value equally remains during one sub-frame. Four phase modulation values acquired during four successive sub-frames are referred to as “phase quadruple”, and there are two types of phase quadruple, such as S1 and S2. Table 2 below shows meanings of S1 and S2. In Table 2, the P-CCPCH is a physical channel into which the BCH corresponding to a transport channel is mapped.
TABLE 2CasePhase quadrupleMeaningS1135, 45, 225, 135The P-CCPCH is in next four sub-frames.S2315, 225, 315, 45The P-CCPCH is not in next four sub-frames.
A frame synchronization acquisition searcher performing the third operation of the initial synchronization acquisition estimates the “phase quadruple” during four sub-frames and determines whether the estimated “phase quadruple” corresponds to S1 or S2. When the estimated “phase quadruple” corresponds to S1, the frame synchronization acquisition searcher determines that there is the P-CCPCH in a next sub-frame and starts receiving the BCH transmitted through the P-CCPCH. At this time, the process of estimating the “phase quadruple” includes a process of estimating a phase modulation value of a downlink SYNC-DL for each sub-frame. A method of the related art for performing the process is largely classified into the following two methods.
(1) A method of applying arctangent to a phase value of an estimated downlink synchronization code and performing hard decision.
(2) A method of calculating a correlation by using estimated phase values of the received mid-amble code and downlink synchronization code and using an inner product with a reference vector.
The method of (1) first estimates the phase value through a complex multiplication of the received downlink synchronization code and the synchronization code already known to the terminal Thereafter, a modulated phase of the downlink synchronization code is calculated by applying arctangent and hard decision is performed. Thereafter, “phase quadruple” corresponding to the modulated phase value during four sub-frames is compared with S1 and S2. As a result of the comparison, when “phase quadruple” corresponds to S1, the BCH is received in four following sub-frames. When “phase quadruple” does not correspond to S1, it is determined that there is no BCH in the four following sub-frames, so that the process of estimating “phase quadruple” is continuously repeated. However, for efficient implementation, the method of estimating the modulated phase value of the downlink synchronization code by using arctangent generally replaces the operation of arctangent with table mapping by a look up table. Accordingly, since a lot of table values should be stored in a memory for the accurate operation, the memory is excessively increased. Further, the modulated phase value of the downlink synchronization code may be distorted by a frequency offset and channel, and significantly degrades accuracy of a result estimated through the method particularly in an environment where a level of the received signal is low. Accordingly, in the method using the phase of the downlink synchronization code by the hard decision, calculation of the accurate phase modulation value is difficult.
The method of (2) uses a correlation value between the received signal and the reference vector by using the reference vector for the phase in order to address the issues of method of (1). Further, the method of (2) performs a frame synchronization estimation in consideration of all available phase modulation possibilities and the frequency offset. Specifically, with respect to M successive sub-frames transmitted from the base station, a phase difference between the downlink synchronization code and the mid-amble code included in each sub-frame is acquired. Thereafter, a complex inner product of the reference vector corresponding to a vector expression of M reference phase values and the phase vector corresponding to a vector expression of phase differences acquired for the M successive sub-frames, and the frame synchronization is estimated using the calculated inner value. When the frame synchronization is estimated by the calculated inner value, a threshold may vary depending on existence or non-existence of the frequency offset or size of the frequency offset.
The method of (2) has a condition based on a frequency offset value generated in a multi-path fading channel, and the frequency offset should be estimated in advance to determine the condition. Further, a frequency offset estimation value should be compared with a reference value. When the estimation value and the reference value are similar to each other, thresholds used herein may be relatively inaccurate values. In addition, when the frequency offset is estimated through a general method in the TDD system, it takes a lot of time corresponding to a minimum of dozens to over one hundred sub-frames according to accuracy of the frequency offset. In analyzing a convergence degree of the frequency offset under various multi-path environments, the frequency offsets converge on significantly different values behind a specific observation section, and one reference value has a difficulty in satisfying the various multi-path environments. Accordingly, “phase quadruple” estimated in an actual environment through the method degrades the accuracy.
Further, the two methods degrade detection capabilities in an area, such as a cell boundary, and have a high probability of generating a false alarm according to the channel and residual frequency offset values. Accordingly, the time corresponding to a minimum of 50 sub-frames should be additionally used for residual frequency offset compensation to successfully decode the BCH.
Therefore, a need exists for a method and an apparatus for acquiring efficient initial frame synchronization and for estimating a residual frequency offset in a CDMA downlink system based on a TDD scheme.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present invention.